I just speak for myself but there can be two(ionic: three) reasons why FileTransfer is undefined. And you do not have to define these objects(FileTransfer, File) on your own, they are defined as soon as you installed both-plugins:

Issue

Good approach:

document.addEventListener("deviceready", onDeviceReady, false);
function onDeviceReady() {
// as soon as this function is called FileTransfer "should" be defined
console.log(FileTransfer);
}